1. Field of the Invention
This invention relates to a magnetic bearing system for a turbo-machine or the like having a relatively lightweight rotor and, particularly, to a magnetic bearing system for supporting such a rotor in a non-contact and stable state.
2. Description of the Prior Art
Magnetic bearing systems for supporting rotors are well known. An example of a conventional magnetic bearing system will be described with reference to FIG. 1.
FIG. 1 is a cross sectional view showing the structure of a spindle supporting apparatus provided with a conventional axis-controlling magnetic bearing system of an inner rotor type. As shown, a rotor shaft 100 is rotatably supported in a casing 101 by a pair of radial magnetic bearings A, A and a thrust magnetic bearing B positioned adjacent to one of the radical magnetic bearings A. The rotor shaft 100 is rotatably driven by a motor M disposed in the mid-portion of the casing 101 and comprising a motor stator 103 and a motor rotor 102. Each of the radial magnetic bearings A includes a radial bearing stator yoke 104 provided with a stator coil 105, a radial bearing yoke 106 mounted on the rotor shaft 100 and radial displacement sensors 107. The thrust magnetic bearing B also includes thrust bearing stator yoke 108 provided with stator coils 109 and a thrust bearing yoke 110 mounted on the rotor shaft 100. Further, the reference numeral 111 in FIG. 1 refers to a rolling bearing for use in an emergency.
The disposition of the magnetic poles of the stators 104 and the displacement sensors 107 is shown in FIGS. 2 and 3. The magnetic attractive forces act in two orthogonally intersecting X- and Y-directions. The position of the rotor shaft 100 in these two axial directions is detected by the displacement sensors 107a and 107b positioned in the X-and Y-directions, respectively, as shown in FIG. 3, and the magnetic attractive forces are controlled on the basis of the signals detected by the sensors. The position in the X-direction of the rotor shaft 100 is regulated by allowing control currents on the basis of the outputs of the displacement sensors 107a in the X-direction to flow through the stator coils 105A and 105C to control the magnetic attractive forces generated between the radial bearing stators 104 and the radial bearing yoke 106.
FIGS. 4 and 5 are block diagrams showing the arrangements of the control circuits for flowing control currents through the stator coil 105A-105D. In the control circuit of FIG. 4, the output from the radial displacement sensor 107 is supplied concurrently with the current from a bias power source 112 through a phase compensating circuit 113 to a power amplifier 114 to permit a control current generated by the power amplifier 114 to flow to the stator coils 105A, 105B, 105C and 105D.
In the control circuit of FIG. 5, the signal supplied from the radial displacement sensor 107 is supplied to the phase compensating circuit 113, the output and inverse output of which is then added to a constant voltage V.sub.R and the output is then respectively input to linear detection circuits 115a and 115c. The outputs thereof are then amplified by power amplifiers 114a and 114c to allow constant currents to flow through the stator coils 105A and 105C.
In the above-described conventional magnetic bearing system, problems are encountered in that the separate provision of the active type radial magnetic bearings A, A and the thrust magnetic bearing B requires an increase in the number of shafts to be controlled in order to support the rotor and in addition, the mounting of the thrust yoke 110 bearing system on the rotor 100 makes assembly and disassembly of the apparatus complicated.
In the magnetic bearings as shown in FIG. 1, in order to linearize between the magnetic attractive force and the current, a constant current is usually caused to flow through each of the stator coils 105A-105D, thereby applying a constant magnetic flux, that is, a biasing magnetic flux is applied between the radial bearing stator 104 and the radial bearing yoke 106. The same can be said with regard to the thrust bearing. As shown in FIG. 2, the magnetic flux .PHI. generated by each of the stator coils 105A-105D passes through a circular magnetic path formed between the radial bearing stator 104 and the radial bearing yoke 106 in a plane perpendicular to the rotor 100 and the stator. As a result, a change in strength of the magnetic flux occurs in the radial bearing yoke 106 in the circumferential direction 1-2-3-4, as shown in FIG. 6. Accordingly, an eddy current is generated on the side of the rotor 100 (the surface of the radial bearing yoke 104) as the rotor 100 rotates across the changing magnetic flux.
Such an eddy current increases as the number of revolutions of the rotor 100 increases. The generation of such an eddy current causes problems of heat generation and internal damping in the rotor 100, thus the rotor fails to be stably supported. A countermeasure generally employed was to use silicon steel plates for the magnetic poles or to reduce the biasing magnetic flux as much as possible in order to minimize the generation of the eddy current, but it has been found that suppression of the eddy current by such a method is still insufficient and that the reduction in the biasing magnetic flux adversely affects the rigidity of the magnetic bearing.